Brake question - odd noise; no pattern

[Guest]

Very occasionally, under light braking, I hear a regular "scrubbing" or "crunching" noise from under the car that sounds like the ABS engaging. There is no pattern to when I hear this noise, other than it is never under heavy braking - only when slowing down gradually. The ABS light does not come on the dash. I think I can feel something like a pulsing in the pedal, but I am not sure because I am always braking lightly. Other than that, there is no other problem . . . no warpage, nothing; and it only happens once in awhile, usually at around-town speeds.

This car is a certified pre-owned and we have warranty through next August, so I am not too concerned, but does anyone have any idea what this might be? The car has 72,000 miles and is otherwise in good condition.